To treat bone fracture or deformation in a human body, two-dimensional fluoroscopic images obtained with X-ray, computed tomography (CT), or perspective images are utilized for making a preoperative planning and performing a bone cutting or correcting operation. However, since the actual deformation of the human bone is three-dimensional, it is difficult to accurately simulate the bone cutting or correcting operation according to the actual situation, before the operation. In the actual practice of the operation, therefore, the bone may be cut at a position deviated from the target position, or the correction may result insufficient. As remedy for such drawback, a bone cutting assist device to be used for operations to treat bone fracture or deformation has been proposed. The assist device is configured to move divided bone pieces obtained by cutting a bone deformed into an abnormal condition to positions for forming the bone of the normal condition. With the device according to Patent Literature (PTL) 1, for example, even an inexperienced surgeon can easily move the divided bone pieces to the target position for the correction, after cutting the bone of an abnormal condition for performing the corrective operation.